Zero Install is a cross-platform, decentralised installation system. Instead of having a central repository in which all software is placed under a naming scheme managed by some central authority, programs and libraries in Zero Install are identified by URIs. Anyone who can create a web-page can publish software. Anyone can install software (not just administrators).
Conceptually, you can imagine that the software is 'installed' only on a remote machine and that you are simply running it (on your own machine) using a network filesystem. Caching makes this fast and allows off-line use. The cache is indexed using a cryptographic hash, allowing it to be shared between mutually untrusting users automatically. Zero Install resolves dependencies (such as libraries) on a per-program basis, which eliminates conflicts. You will never be told that installing one program requires first uninstalling another.